Review on Fastcall by Devin Girard

Great solution for all types of call center, including mobile phone, VoIP, SIP and UC solutions. You get full control over the user experience and don't have to integrate with any 3rd party systems. In addition, you can generate detailed analytics, so you'll know exactly who called and when. I am impressed by how fast they respond and help solve issues. They really make sure your problems are solved quickly. Also, if there is an update to the API or product, it's done very transparently and will allow users to continue using the application without downtime.